EL NIÑO AND LA NIÑA ACROSS THE CENTRAL CALIFORNIA COASTAL UPWELLING ZONE: PHYSICS, NUTRIENTS, AND EFFECTS ON PHYTOPLANKTON

EL NIÑO AND LA NIÑA ACROSS THE CENTRAL CALIFORNIA COASTAL UPWELLING ZONE: PHYSICS, NUTRIENTS, AND EFFECTS ON PHYTOPLANKTON

EL NIÑO AND LA NIÑA ACROSS THE CENTRAL CALIFORNIA COASTAL UPWELLING ZONE: PHYSICS, NUTRIENTS, AND EFFECTS ON PHYTOPLANKTON

**J. T. Pennington, R. P. Michisaki, C. G. Castro , F. P. Chavez

**MBARI, Moss Landing, CA 95039 USA, Naval Postgraduate School, Monterey, CA USA

**J. T. Pennington, R. P. Michisaki, C. G. Castro , F. P. Chavez

**MBARI, Moss Landing, CA 95039 USA, Naval Postgraduate School, Monterey, CA USA

SUMMARY OF CHEMICAL AND BIOLOGICALEFFECTS

Depth-integrated NO3, chlorophyll and primary productionvalues all show the effects of these events across Line 67 (Fig.4). With the onset of El Niño in summer 1997, the productiveupwelling system collapsed to shore and did not redevelop inspring and summer of 1998, leading to near-zero values for all3 parameters, particularly offshore. With the onset of La Niñain late 1998, the productive zone spread at least 300 kmoffshore. 1999, with stronger La Niña conditions, was moreproductive than 2000.

When the depth-integrated values are differenced fromclimatology, overall effects of El Niño and La Niña along Line67 can be assessed (Fig. 5). It turns out that while El Niño hadstrong negative impacts on integated nitrate, chlorophyll andprimary production across Line 67, La Niña’s presumedpositive effects are weak and/or mixed when viewed in thisway. We have several working hypotheses which may accountfor this apparent lack of effect: (1) the 240 km on/offshorespatial domain is too broad, and La Niña’s effects wereaveraged away; (2) La Niña’s effects are strongest in winter,when its effects on phytoplankton are minor due to lightlimitation and mixing (this argument fails, however, for ElNiño); (3) lack of successful cruises during the two 1999seasons prevents us from seeing an effect; and (4) our 1989-91climatology may include several cold La Niña-like years,aliasing calculated anomalies. We are now exploring thesehypotheses.

ACKNOWLEDGEMENTS

The David and Lucile Packard Foundation funded thework reported here. We thank the many scientists whoparticipated and the crews of the R/Vs Point Sur and NewHorizon.

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CARBON

EL NIÑO AND LA NIÑA OFF CENTRAL CALIFORNIA

The 1997-98 El Niño began in the equatorial Pacific in spring 1997, persisted into summer 1998, and was followed by the still-continuing La Niña (Fig. 1, top panel). In our central California study area, El Niño was reflected in anomalously high sea levels particularly in winter, high SSTs and a deep thermocline, and below-normal upwelling indices during the latter part of the event (spring 1998; Fig. 1, bottom 3 panels). La Niña's signature on these variables has been characterized by low sea levels again in winter, lower SSTs anda shallow thermocline, and consistently above normal upwelling indices during the early part of the event (spring/summer 1999; Fig. 1, bottom 3 panels) . .In this presentation we describe El Niño and La Niña as observed during cruisework off central California, with emphasis on phyto-plankton distribution and production. The data were collected during an ongoing cruise series which occupies an onshore/offshore transect extending across the coastal upwelling zone into the California Cur ren t . . CRUISES AND RESULTS . Twenty-two cruises have been conducted approximately quarterly from 1997 to the present ('SECRET' cruise series --- Studies in Ecological and Chemical Responses to Environmental Trends) along the nearshore 275 km segment of CalCOFI Line 67 (Fig. 2). Eleven stations are occupied and routine physical, chemical and biological data are collected. In addition, data from 18 cruises conducted during 1989-91 also along Line 67 ('PEGASUS' cruise series) have been taken as a representative climatology. .

Detailed results are presented as contours of basic oceanographic st

variables (Fig. 3). The 1 row of contours for each variable depicts the climatological (1998-91) values for each season beginning with the

ndfall or 'Oceanic' season. The 2 row represents the distribution during rd th

the 4 seasons of the 1997-98 El Niño; and the 3 and 4 rows represent the distributions during the 2 years of La Niña (1998-present).

El Niño, Oceanic 1997 - Late Upwelling 1998. El Niño first became obvious at the surface in summer 1997. Relative to climatology, SST's were very warm and the thermocline deep; some coastal upwelling was evident in the temperature contours during the upwelling season of 1998. The salinity contours are similar to temperature, except that a prominent shallow layer of fresh water appeared during the Early Upwelling season of 1998. This was apparently caused by coastal runoff and/or an onshore excursion of a very low salinity core of the California Current. This spring was additionally characterized by low upwelling indices (Fig. 1, bottom panel). Surface nitrate values were near zero across the transectthroughout the El Niño, with some minimal elevation of values nearshore. Chlorophyll and primary production values were similarly depressed, again with some minimal elevation nearshore.

La Niña, Davidson 1998 - Late Upwelling 1999. Unusual outcropping of isotherms and isohalines occurred nearshore during the Davidson season of 1998-99, even though the Upwelling Index was only slightly elevated during this season (Fig. 1, bottom panel). However the Upwelling Index rose dramatically in spring and summer 1999 and the pattern of strong shoreward shoaling of isolines (Fig. 3) continued atleast through the Late Upwelling season of 1999. High surface NO3 values persisted both near- and offshore through at least the Late Upwelling season of 1999, and as a consequence higher chlorophyll values were broadly distributed offshore (see also Fig. 4). Primary production values were also fairly high relative during this period.However, relative to climatology, the effects of La Niña have not been nearly as dramatic as those of El Niño. .

Figure 5. Seasonal anomalies of depth-integrated nitrate, chlorophyll and primary production. Depth-integrated values were averaged across the nearshore 240 km of Line 67. The El Niño and La Niña seasons were differenced from the 1989-91 climatological values to produce seasonal anomalies across the transect. Seasons are Early Upwelling (EU)= Feb.,Mar., Apr., Late Upwelling (LU) = May, Jun., Jul. Oceanic (OC) = Aug., Sep., Oct. Davidson (DV) = Nov., Dec., Jan. Top panel: Absolute values in metric tons. Bottom panel: Percent difference from climatology. .

Figure 1. The time courses of El Niño and La Niña, in the equatorial Pacific and off central California. Dotted vertical lines indicate El Niño and La Niña periods covered by Figure 3. Top panel: The multivariate ENSO index, with positive values indicating El Niño and negative values indicating La Niña conditions on the equator. Second panel: Adjusted sea levels in Monterey Harbor --- high sea levels indicate onshore and/or poleward flow (El Niño); low sea levels should indicate the converse (La Niña). Third panel: Time/depth contours of temperature from the MBARI M1 mooring in central Monterey Bay. El Niño is indicated by warm SSTs and a deep thermocline; La Niña has been characterized by colder SSTs and water column. Bottom panel: The upwelling index (blue) and its departures from climatology (red) for central California, indicative of local winds. Negative anomalies should favor local El Niño conditions; positive anomalies should favor local La Niña conditions.

Figure 2. Map of the central California study region showingthe onshore/offshore transect (CalCOFI Line 67) used during both the SECRET and PEGASUS cruise series'. Eleven stations to 275 km offshore have been occupied quarterly since 1997.

Figure 4. 60-m integrated nitrate, and water-column integrated chlorophyll and primary production as time/distance-offshore contours. The 1997-98 El Niño is visible as the white region of very low values offshore. Conversely, La Niña is the broad blue region of high values particularly well-developed during spring and summer of 1999. .

stFigure 3. Temperature, salinity, nitrate, chlorophyll, and primary production contours. Data from the cruise series' have been lumped into 4 'oceanographic seasons' (as noted over each contour plot) and distance-offshore/depth contour plots constructed. The 1 row of contours for each variable depicts the climatologicalnd rd th

(1998-91) values for each season beginning with the fall or 'Oceanic' season. The 2 row represents the distribution during the 4 seasons of the 1997-98 El Niño; and the 3 and 4 rows represent the distributions during the 2 years of La Niña (1998-present). The abbreviated contours during the Early Upwelling season of 1999 are due to a very rough cruise where it was not possible to occupy offshore stations; no cruise was conducted during the Oceanic season of 1999. Seasons are Early Upwelling (EU)= Feb., Mar., Apr., Late Upwelling (LU) = May, Jun., Jul. Oceanic (OC) = Aug., Sep., Oct. Davidson (DV) = Nov., Dec., Jan. .

La Niña, Oceanic 1999 - Late Upwelling 2000. Temperature and nd

salinity signatures were not striking during this 2 La Niña year, although unusual shoreward outcropping of isolines again occurred during the Davidson season. NO3, chlorophyll and primary

ndproduction values were also not strikingly different during this 2 year from climatological values. .

ASLO PC18 Biogeochemical Cycles